1. Field of the Invention
This invention relates to electronic devices implantable within the human body and in particuilar to apparatus for monitoring the heart's activity.
2. Description of the Prior Art
Heart pacemakers such as that described in U.S. Pat. No. 3,057,356 issued in the name of Wilson Greatbatch and assigned to the assignee of this invention, are known for providing electrical stimulus to the heart, whereby it is contracted at a desired rate in the order of 72 beats per minute. Such a heart pacemaker is capable of being implanted in the human body and operative in such an environment for relatively long periods of time. Typically, such pacemakers are implanted within the chest beneath the patient's skin and above the pectoral muscles or in the abdominal region by a surgical procedure wherein an incision is made in the selected region and the pacemaker is implanted within the patient's body. Such a pacemaker provides cardiac stimulation at low power levels by utilizing a small, completely implanted transistorized, battery-operated pacemaker connected via flexible electrode wires directly to the myocardium or heart muscle. The electrical stimulation provided by this pacemaker is provided at a fixed rate.
In an article by D. A. Nathan, S. Center, C. Y. Wu and W. Keller, "An Implantable Synchronous Pacemaker for the Long Term Correction of Complete Heart Block", American Journal of Cardiology, 11:362, there is described an implantable cardiac pacemaker whose rate is dependent upon the rate of the heart's natural pacemaker and which operates to detect the heart beat signal as derived from the auricular sensor electrode and, after a suitable delay and amplification, delivers a corresponding stimulus to the myocardium and in particular, the ventricle to initiate each heart contraction.
Such cardiac pacemakers, separately or in combination, tend to alleviate some examples of complete heart block. In a heart block, the normal electrical interconnection in the heart between its atrium and its ventricle is interrupted whereby the normal command signals directed by the atrium to the ventricle are interrupted with the ventricle contracting and expanding at its own intrinsic rate in the order of 30-40 beats per minute. Since the ventricle serves to pump the greater portion of blood through the arterial system, such a low rate does not provide sufficient blood supply. In normal heart operation, there is a natural sequence between the atrial contraction and the ventricular contraction, one following the other. In heart block, there is an obstruction to the electrical signal due, perhaps, to a deterioration of the heart muscle or to scar tissue as a result of surgery, whereby a block in the nature of a high electrical impedance is imposed in the electrical flow from the atrium to the ventrical.
Where the heart block is not complete, the heart may periodically operate for a period of time thus competing for control with the stimulation provided by the artificial cardiac pacemaker. Potentially dangerous situations may arise when an electronic pacemaker stimulation falls into the "T" wave portion of each natural complete beat. As shown in FIG. 1, the "T" wave follows by about 0.2 seconds each major beat pulse (or "R" wave causing contraction of the ventricles of the heart). Within the "T" wave is a critical interval known as the "vulnerable period" and, in the case of a highly abnormal heart, a pacemaker impulse falling into this period can conceivably elicit bursts of tachylcardia or fibrillation, which are undesirable and may even lead to a fatal sequence of arrhythmias.
Cardiac pacemakers of the demand type are known in the prior art such as that disclosed by United Kingdom Pat. No. 826,766 which provides electrical pulses to stimulate the heart only in the absence of normal heartbeat. As disclosed, the heartbeat is sensed by an acoustical device disposed external of the patient's body, responding to the presence of a heartbeat to provide an inhibit signal defeating the generation of heart stimulating pulses by the pacemaker. In the absence of the patient's natural heartbeat, there is disclosed that the pacemaker generates pulses at a fixed frequency.
In U.S. Pat. No. Re. 28,003, of David H. Gobel, assigned to the assignee of this invention, there is disclosed an implantable demand cardiac pacemaker comprising an oscillator circit for generating a series of periodic pulses to be applied via a stimulator electrode to the ventricle of the heart. The stimulator electrode is also used to sense the "R" wave of the heart, as derived from its ventricle to be applied to a sensing portion of the cardiac pacemaker wherein, if the sensed signal is above a predetermined threshold level, a corresponding output is applied to an oscillator circuit to inhibit the generation of the stimulator pulse and to reset the oscillator to initiate timing a new period. The following patents, each assigned to the assignee of this invention, provide further examples of demand type heart pacemakers: U.S. Pat. No. 3,648,707 of Wilson Greatbatch; U.S. Pat. No. 3,911,929 of David H. Gobeli; U.S. Pat. No. 3,927,677 of David H. Gobeli et al; U.S. Pat. 3,999,556 of Clifton Alferness; and U.S. Pat. No. 3,999,557 of Paul Citron et al.
Demand type pacemakers are particularly adapted to be used in patients having known heart problems such as arrhythmias. For example, if such a patient's heart develops an arrhythmia, failing to beat or to beat at a rate lower than a desired minimum, the demand type pacemaker is activated to pace the patient's heart at the desired rate. Of particular interest to the subject invention, are those patients that have recently undergone heart surgery; typically, these patients are apt to develop any and all known arrhythmias in the immediate post-operative period. Current therapy for such patients involves the implanting at the time of surgery of cardiac leads with their electrodes connected to the patient's heart and the other ends of the leads being connected to an external pacemaker to provide pacing for arrhythmia management.
In addition, the same pacemaker leads that interconnect the internally planted electrodes and its external pacemaker, are also connected to an external monitoring unit for providing signals indicative of the patient's heart activity to the external monitoring unit. A significant advantage of such pacemaker leads is that they may be used for recording of direct epicardial electrograms, which provide high quality precision data as to the patient's heart activity. The study of such wave shapes, i.e., morphology, is an invaluable aid in a diagnosis of arrhythmias. In this regard, it is understood that a normal EKG having its electrodes attached to various portions of the patient's skin does not provide the high quality output signal for diagnosis of arrhythmias as is obtained by cardiac electrodes attached directly to a patient's heart. For example, the output signal as obtained from such directly attached electrodes has a bandwidth in the order of 500 Hz and a signal to noise ratio in the order of 40 to 1, with no more than 30 db frequency loss. Such a high quality EKG signal cannot be obtained from a standard EKG monitor as is attached only to the outer skin of the patient.
However, the use of pacemaker leads directed through the patient's skin presents certain problems. Typically, if the external leads are left in the patient for any length of time, e.g., 5 to 7 days, an infection may develop at the exit side of the leads, and the leads may be accidentally pulled with subsequent damage to the patient's heart. Further, such leads present micro and macro shock hazards to the patient. For example, there are small residual charges on many objects within a surgical environment and if the leads are accidentally exposed to such a charge, it will be applied via the leads to the patient's heart possibly inducing an arrhythmia therein. Further, relatively high voltage such as carried by an AC powerline are typically found in the operating room; the electrogram recording apparatus is so powered and the contemplated accidental contact of the external leads with such an AC powered line would have serious consequences for the patient. In addition, it is necessary to remove the cardiac leads approximately 5 to 7 days after their surgical implantation. Further, there is considerable electrical environmental noise within an intensive care unit where a post-operative cardiac patient would be placed. Illustratively, such noise results from fluorescent lights or other electrical equipment typically found in an intensive care unit and is capable of inducing millivolt signals into such cardiac leads of similar amplitude to those signals derived from the patient's heart. Thus, such environmental noise-induced signals may serve to inhibit the external pacemaker from pacing, even though the patient's heart may not be beating. Further, it is contemplated that after the surgical implantation of such demand pacemakers, that the connections of the atrial and ventrical leads to the external pacemaker may be reversed, with resulting pacer-induced arrhythmias.
The prior art has suggested artificial pacemakers having a transmitter or unit disposed externally of the patient's body and a receiver surgically implanted within the patient, having leads directly connected to the patient's heart. For example, in the West German Auslegeschrift No. 25 20 387, entitled Testing Arrangement for Artificial Pacemakers, there is described a pacemaker having an external transmitter for transmitting external energy by radio frequency (RF) waves to an internally planted unit for supplying electrical stimulation to the heart. Further, it is disclosed that the internally planted unit is capable of transmitting information to a monitoring device disposed externally of the patient's body, for indicating various characteristics of the pacemaker.
Further, in a pair of articles entitled "A Demand Radio Frequency Cardiac Pacemaker", by W. G. Holcomb et al, appearing in Med. & Biol. Eng., Vol. VII, pp. 493-499, Pergamon Press, 1969, and "An Endocardio Demand (P&R) Radio Frequency Pacemaker", by W. G. Holcomb et al, appearing in the 21st ACEMB, page 22A1, November 18-21, 1968, there is described a demand-pacemaker including an external transmitter 10' as shown in FIG. 2, labeled PRIOR ART, for generating an RF signal from its primary coil or antenna 16' to be received by a receiver 12' internally implanted within the patient's skin 14'. In addition, the receiver 12' in turn transmits heart activity in terms of the currents of the heart's "R" wave to synchronize the activity of a pulse generator 26 within the external transmitter 10'. As shown in FIG. 2, the receiver 12' includes two separate electronic circuits each sharing common leads connected to the pacemaker electrodes, which are surgically connected to the patient's heart. The first circuit, i.e., the EKG transmitter section, consists of a rectifying circuit of diodes D20-D23 for providing power to a transistor amplifier Q10, to which is applied the EKG signal; the amplified EKG signal is applied in turn to a coil 34'a for transmission to the transmitter 10'. The primary coil or antenna 16' receives and applies the EKG signal via a detector 30, to be amplified by an amplifier 30, which provides the indicated EKG signal to be analyzed upon a display not shown. The second electronic circuit of the receiver 12' is the stimulus receiver, which furnishes the stimulating pulse to the cardiac electrodes. In particular, the output of the pulse generator 26 of the transmitter 10' is applied via closed switch 24 to superimpose a high voltage pulse upon the output of the 2 MHz oscillator, which is subsequently amplified by amplifier 20 and applied via detector 18 to the antenna 16'. The high voltage pacemaker pulse as superimposed upon the RF carrier, is received by the coil 34'b and rectified by the diode D25 and the capacitor C25 to actuate an electronic switch primarily comprised of transistors Q12 and Q13, which are closed thereby to apply the high voltage pulse via FET Q11 to the pacemaker electrodes, the FET Q11 serving to regulate the current passing to the pacemaker electrodes. The transistors Q12 and Q13 are voltage-responsive and disconnect the coil 34'b from the pacemaker electrodes in the absence of the high voltage pacemaker pulse.
It is understood that the RF carrier as derived from the oscillator 22 is continuously applied to the coil 16'. The secondary coil 34'a receives a continuous RF wave from the primary coil 16'. An EKG signal is derived from the pacemaker electrodes and is applied to the base of the amplifier transistor Q10, which in turn provides a correspondingly varying load to the coil 34'a, whereby a corresponding voltage fluctuation is induced across the coil 16'. In other words, the voltage appearing across the coil 16' is amplitude modulated in accordance with the patient's heart activity or EKG signal. Though the circuitry shown in FIG. 2 provides a relatively simple circuit of energizing the receiver 12' implanted within the patient, the EKG signal as derived from the patient does not contain sufficient precision to provide a diagnostic quality display of the patient's EKG. Typically, to provide a diagnostic quality display of the patient's EKG it is necessary to transmit the EKG signal with a bandwidth of 100 Hz with a signal to noise ratio in thr order of 40 to 1 and with no more than a 3 db frequency loss; the circuitry shown in FIG. 2 does not provide such quality primarily due to the amplitude modulation type of signal transmission, which is sensitive to the relative positions in terms of distance and angle of orientation between the coils 16' and 34'a. In this regard, if the distance or the angle between the axes of the coils 16' and 34'a vary due to the patient's movement, the amplitude of the signal as seen by the detector 18 also will vary. This, in an amplitude modulation system, this body movement will provide a distortion in the EKG signal detected. In addition, the extraneous noise to which such a pacemaker would be exposed such as radiation from fluorescent lights or AC power lines, as well as other extraneous artifacts, may appear as amplitude modulation to introduce further errors in the signal received from the transmitter 10'.